Synthesis and Structure of PNP-Supported Iridium Silyl and Silylene Complexes: Catalytic Hydrosilation of Alkenes

摘要

Oxidative addition of bulky primary, secondary, and tertiary silanes to PNP (PNP = [N(2-P'Pr_2-4-Me-C_6H_3)_2]~-) iridium complexes (PNP)lrH_2 and (PNP)lr(COE) (11) afforded iridium silyl hydride complexes (PNP)lr(H)(SiRR'R") (3-8). Addition of 2 equiv of PhSiH_3 or (3,5-Me_2C_6H_3)SiH_3 to (PNP)lrH_2 or 11 yielded disilyl complexes (PNP)lr(SiH_2R)_2 (R = Ph (9), 3,5-Me_2C_6H_3 (10)). Hydride abstraction from (PNP)lr-(H)(SiH_2R) (R = Trip (5), Dmp (6)) by [Ph_3C][B(C_6F_5)_4] afforded iridium silylene complexes [(PNP)(H)lr= SiR(H)][B(C_6F_5)_4] (R = Trip (12), Dmp (13)) exhibiting downfield ~(29)Si NMR resonances (234 ppm (12), 226 ppm (13)) and downfield 1H NMR resonances for the Si-H group (10.76 ppm (12), 9.76 ppm (13)). Thermally stable disubstituted silylene complexes [(PNP)(H)lr=SiPh_2][A] (A = -R(C_6F_5)_4 (14), ~-CB_(11)H_6Br_6 (16)) were isolated via hydride abstraction from (PNP)lr(H)(SiHPh_2). The X-ray structure of 16 confirmed sp~2 hybridization at silicon and revealed a short Ir-Si bond of 2.210(2) A. Catalytic hydrosilation of alkenes by hydrogen-substituted silylene complexes [(PNP)(H)lr=SiMes(H)][B(C_6F_5)_4] (1) and 14 exhibited anti-Markovnikov regioselectivity with an array of alkene substrates. Addition of H3SiMes to complex 1 afforded [(PNP)(SiH(Mes)(Hex))lrH(SiH_2Mes)][B(C_6F_5)_4] (19), featuring a β-agostic interaction demonstrated by a J_(SiH) of 102 Hz for the N-SiH hydrogen. Similarly, addition of H_2SiPh_2 to 16 afforded the structurally characterized lr(V) disilyl complex [(PNP)(SiPh_2)lr(SiPh_2H)(H)_2][CB_(11)H_6Br_6] (20). Complex 20 was found to be catalytically active for the hydrosilation of alkenes, which is consistent with its intermediacy in the catalytic cycle.